1. Field of the Invention
The present invention relates to an image heating apparatus, adapted for use as a toner image fixing apparatus in an image forming apparatus such as a copying apparatus, a laser beam printer, a facsimile or the like.
More specifically, the present invention relates to an image heating apparatus adapted for use as a fixing apparatus in a process of forming an unfixed toner image corresponding to desired image information on a recording medium (paper, printing paper, transfer sheet, a transparent sheet for an overhead projector (hereinafter referred to as an OHT sheet), glossy paper, glossy film etc.) either by a direct method or an indirect (transfer) method, utilizing a toner constituted for example of heat fusible resin and employing image forming process such as electrophotography, electrostatic recording or magnetic recording, and heat fixing such unfixed toner image as a permanent fixed image on the recording medium bearing such image.
2. Related Background Art
In an image forming apparatus such as a printer or a copying apparatus, image formation is often executed by an electrophotographic process, and, in such process, a toner image is formed on a recording medium by a transfer method or a direct method, and is fixed by applying heat and pressure to such recording medium.
A fixing apparatus for fixing the toner image has conventionally employed a heat roller system. This system is basically constituted of a metal roller provided therein with a heater, and an elastic pressure roller pressed thereto, and a recording medium which is a member to be heated is introduced into a nip portion of the paired rollers, and pinched and passed therein to heat the toner image under heat and pressure.
However the fixing apparatus of such heat roller type requires a very long time for elevating the roller surface to a fixing temperature, because of a large heat capacity of the roller. For this reason, in order to achieve a prompt image outputting operation, the roller surface has to be controlled at a certain temperature even while the apparatus is not in use.
Therefore, Japanese Patent Application Laid-open No. H4-44075 teaches an image heating apparatus of a film heating type. Such image heating apparatus is generally constituted of a thin heat-resistant film, a heater fixed at the side of a surface of the film, and a pressure roller provided at the other surface and opposed to the heater for contacting the recording medium to be heated to the heater through the film. When such apparatus is employed as a fixing apparatus, a recording medium on which a toner image is formed is introduced into and passed by a nip portion formed by the heater and the pressure roller, with the film interposed, whereby the surface bearing a visualized image on the recording medium is heated by the heater through the film to give the unfixed image with thermal energy and the image is heat fixed by softening and fusing of the toner.
The image heating apparatus of the aforementioned film heating type, capable of employing a heater of a low heat capacity as the heating member, can achieve an electric power saving and a shorter wait time in comparison with the prior apparatus of a heat roller type or a belt heating type.
However, the image heating apparatus of film heating type, utilizing a polyimide film or the like having a very small heat capacity, has a limitation in the fixing property or the glossiness for use in an image forming apparatus requiring a high process speed or a color image fixation.
It is therefore strongly desired to enable a high speed drive or an adaptation to a color image forming apparatus, while retaining advantages of inexpensiveness and absence of warm-up operation in the image heating apparatus of film heating type. A heat conductivity of the film is important in achieving a higher speed. In order to transmit the heat of the heater promptly to the recording medium, the film has to be constituted of a material of a thermal conductivity as high as possible. Also in order to adapt to a color image formation, the film has to be provided with an elastic layer. In a film provided with an elastic layer, when a toner image passes through the fixing nip, the elastic layer is deformed according to the toner layer whereby the toner is enclosed by the elastic layer and is uniformly heated to achieve a uniform fixation. Such uniformly fixed image is free from unevenness in luster and shows an excellent transmission of the image particularly in case of a fixation of an OHT sheet.
For enabling a higher speed or a color image formation as explained above, Japanese Patent Application Laid-open Nos. 2003-45615 and 2003-156954 propose an image heating apparatus of film heating type employing a metal of higher thermal conductivity such as stainless steel or nickel instead of usual resinous material for a base material of the film.
However, in case of employing a metal film, it is charged for example by a frictional charging by a conveyed material to be heated, thereby leading, in case of a fixing apparatus, to an uneven gloss or an image defect because of phenomena that the unfixed toner is scattered from the recording medium to be heated or is attracted to the film surface. Also the accumulated charge, if released at once at a certain timing, causes a noise to a signal system and may induces an erroneous operation in the main body of the apparatus.
Also in case a heating member, in sliding contact with the metal film, is broken by any reason, an electric current may leak from such broken heating member to a main frame of the image forming apparatus through the metal film, thereby eventually resulting in a danger of electric shock.